The present invention relates to a wind turbine and to a method of operating a wind turbine that prevents excessive loads acting on the rotor blades.
Wind turbine rotor blades experience a significant level of dynamic loading during operation of the wind turbine. In part, this dynamic loading is exerted by the wind, and in particular, excessive transient loads can be caused by turbulence or gusts i.e. short periods of very high wind. Wind loading may cause extreme deflection of the blades and can result in severe stresses in the blades. Over the lifetime of the wind turbine (potentially 20 years or more), cyclic stress may cause the rotor blades to fail due to material fatigue.
The rotor blades must be robust and reliable in order to reduce the potential maintenance time and costs over the lifetime of the wind turbine. Therefore, modern wind turbines include a number of load sensors such as strain gauges for measuring the strain on the rotor blades. The measured strain on the rotor blades can be used to determine the stress experienced by the rotor blades through predetermined stress-strain relationships linked to the material and construction of the rotor blades. To prevent high stresses, the pitch of the rotor blades may be varied or the shape of the cross section of the rotor blades may be changed (e.g. using flaps). These protective actions are intended to reduce the wind loading on the rotor blades and hence reduce stresses in the rotor blades.
Generally, the strain gauges are fitted at the root end of the rotor blades, i.e. close to where the rotor blade is mounted to the hub of the wind turbine. A maximum strain limit is typically imposed such that if the measured strain reaches that limit, protective action is taken (such as varying the pitch or camber of the rotor blades) to reduce fatigue on the rotor blade and to prolong its life. However, this also has the adverse effect of reducing the overall energy capture by the wind turbine during periods of high wind.
In order to avoid damage to the blades caused by excessive loads, some wind turbines operate below a so-called ‘safety strain limit’, which is considerably lower than the maximum strain limit that the blades can tolerate without being damaged. In this case, protective actions to reduce the load on the blades are taken when the safety strain limit is reached (i.e. before the maximum strain limit is reached). This safety strain limit therefore provides a safety margin in which the wind turbine can operate without the possibility of the blades being damaged. However, if the blades are exposed to steady high winds without gusts or turbulence, the safety strain limit may be exceeded and protective actions taken, even though the steady wind loads would not cause the blades to exceed their maximum strain limit. In such cases, there is no risk of damage occurring, but the protective actions inevitably result in reduced energy capture.
There is a continual drive to increase the size and efficiency of wind turbines. As rotor blades become larger, they also become more flexible and are exposed to greater wind forces. There is therefore a need to develop increasingly-more sophisticated load monitoring and control systems for wind turbines that are capable of preventing damage from occurring to the blades whilst at the same time maximising the energy capture from the wind.